The main function of a power supply arranged in a computer is firstly converting an AC power to a DC power, and then providing the DC power to a motherboard or other peripheral devices. Referring to FIG. 1, a conventional power supply system providing specific voltages to a motherboard is shown. The system comprises a power supply 80 and a motherboard 90. According to specification of the computer power supply system, the power supply 80 further comprises 24 pins, 4 pins, VGA pins, and HD pins. The 24 pins, the 4 pins, the VGA pins, and the HD pins are connected to a 24-pin plug 82, a 4-pin plug 84, a VGA plug 86, and a HD plug 88, respectively. The motherboard 90 further comprises a 24-pin jack 92, a 4-pin jack 94, a VGA jack 96, and a HD jack 98; where the 24-pin jack 92, the 4-pin jack 94, the VGA jack 96, and the HD jack 98 are corresponding to the 24-pin plug 82, the 4-pin plug 84, the VGA plug 86, and the HD plug 88, respectively. After the connected between the plugs of the power supply 80 and the corresponding jacks of the motherboard 90, the specific voltages (+3V, +5V, +5 VSB (Stand-By-Power), and +12V) can be transmitted from the power supply 80 to the motherboard 90.
However, because user may not switch off the AC power of the power supply after the shutdown of the computer, the Stand-By-Power (e.g., +5 VSB) is kept providing to the motherboard. In other words, the Stand-By-Power (e.g., +5 VSB) is always outputted from the power supply 80 to the motherboard 90 if the computer system is off but the AC power is connected to the power supply 80. After a long-term period, the circuit related to the Stand-By-Power in the power supply 80 accordingly may be easier damaged than other circuit parts in the power supply 80. Once a portion of the power supply 80 is damaged, the computer system cannot be power on successfully.